Null dereference in print_odf_element function

## Problem Description

Note: I am aware that this project is unmaintained. However, I am still opening this issue to follow [CVE's guidelines for EOL software](https://www.cve.org/Resources/General/End-of-Life-EOL-Assignment-Process.pdf).

There is a null dereference in the `print_odf_element` function when the parser handles a specially crafted Markdown file. This could be used to perform a denial-of-service attack.

Here is a minimized proof-of-concept Markdown file that triggers the bug: [min_null_deref_print_odf_element.md](https://github.com/jgm/peg-markdown/files/12413267/min_null_deref_print_odf_element.md).

The output is as follows:

```
AddressSanitizer:DEADLYSIGNAL
=================================================================
==761952==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x0000005db3b7 bp 0x000000000014 sp 0x7fffffffe030 T0)
==761952==The signal is caused by a READ memory access.
==761952==Hint: address points to the zero page.
#0 0x5db3b7 in print_odf_element /home/sanic/peg-markdown/markdown_output.c:1072:42
#1 0x5dac3a in print_odf_element_list /home/sanic/peg-markdown/markdown_output.c:892:9
#2 0x5dac3a in print_odf_element /home/sanic/peg-markdown/markdown_output.c:1054:9
#3 0x5d4c0a in print_odf_element_list /home/sanic/peg-markdown/markdown_output.c:892:9
#4 0x5d4c0a in print_element_list /home/sanic/peg-markdown/markdown_output.c:1146:26
#5 0x5dca53 in markdown_to_g_string /home/sanic/peg-markdown/markdown_lib.c:163:5
#6 0x5dce84 in markdown_to_string /home/sanic/peg-markdown/markdown_lib.c:175:11
#7 0x4c4d06 in main /home/sanic/peg-markdown/markdown.c:180:11
#8 0x7ffff7b25082 in __libc_start_main /build/glibc-SzIz7B/glibc-2.31/csu/../csu/libc-start.c:308:16
#9 0x41c50d in _start (/home/sanic/peg-markdown/markdown+0x41c50d)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV /home/sanic/peg-markdown/markdown_output.c:1072:42 in print_odf_element
==761952==ABORTING
```

## Reproduction Steps
1. Compile the project using ASAN (Address Sanitizer). For example, `CC=afl-clang-fast AFL_USE_ASAN=1 make`.
2. Run `./markdown --to=odf min_null_deref_print_odf_element.md` (use the proof-of-concept file attached to this report).
3. Observe the null dereference in the output.